Does your car has the steel strap that holds the front of the seat support down?

I have seen several 544s without them.

I would move the seat rails rearward and add some support to the rear of the seat. Perhaps a wooden block about 1 inch thick with a 3 inch wide steel piece to spread the weight out on the floor. An inverted T.

The support may not have to go past the rear of he seat bottom.

Your rear seat passengers may have to keep their feet between the rails.

I have 850 seats in my 544 mounted on an adaptor plate bolted to the original floor mounts.